This invention relates to a core for molds for the electrical melting of metals to form hollow ingots, especially for the slag-shielded electrical melting of consumable electrodes, having a wall whose outside dimensions correspond to those of the hollow of the ingot.
The production of hollow ingots, especially by the slag-shielded electrical melting of consumable electrodes, has long been known. In contrast to the method of producing a solid cast ingot and then perforating it, hollow ingot casting has the advantage of a virtually complete utilization of the material and the formation of a high-quality, fine-grain structure substantially free of voids and segregations. The reason for this lies in the short distance between every element of the volume of the hollow ingot and the cooled mold walls, i.e., in a favorable ratio of the internal and external surface area of the hollow ingot to the volume thereof. For the manufacture of hollow bodies such as tanks, hoops or tubes it would therefore be desirable to cast them directly as hollow bodies.
To produce a hollow ingot, it is necessary to provide within the mold a core which during the production of the ingot is withdrawn from the ingot or from the portion thereof which has solidified. A very serious problem is the unavoidable shrinkage of the ingot during cooling, which entails the danger that the ingot may shrink tight on the core.
In casting processes modeled on continuous strand casting, in which a mold with a core fastened therein moves relative to the hollow ingot, there is the danger that the core might "freeze" in the ingot, bringing the entire casting process to a halt and necessitation shut-down. In casting processes using a stationary mold and stationary core, there is the danger that the ingot may shrink tight on the core such that it can no longer be released undestructively from the ingot.
To obviate the problems described above, it has long been known to make the core of foundry sand, and after solidification and cooling of the ingot, to remove the core by destroying it. Such a sand core, however, has the disadvantage of poor thermal conductivity and therefore of an unfavorable influence on grain structure. Especially, however, it is not sufficiently resistant to a superheated molten metal, and particularly to molten slag, which has the tendency to dissolve ceramic compositions and hence also foundry sand. Inclusions of particles of foundry sand in the hollow ingot are especially dangerous.
It is consequently necessary to use metal cores, which must necessarily be liquid cooled. In order in this case to prevent the hollow ingot from shrinking tight on the core, attempts have already been made to make the wall of the core a corrugated wall having corrugations running parallel to the long axis of the core, so as to provide it to some extent with an elastic compressibility. In this case, however, a corresponding corrugated profile is necessarily formed on the inside of the hollow ingot, and has to be removed by a difficult working procedure. Furthermore, the ingot fills the grooves between the corrugations of the core, so that the radial compressibility is largely lost. Such attempts, therefore, have not resulted in success.
Attempts have furthermore been made to make only the surface of a metal, water-cooled core compressible radially. For this purpose it has become known to coat the core surface with a compressible, inorganic layer of material of good thermal conductivity, although the selection of the material has been left open. Thus far no material has become known which even remotely corresponds to these requirements. It must be considered that the nature of such a substance excludes good thermal conductivity. Furthermore, as already stated above, virtually all ceramic materials are attacked by hot, molten slag, so that a core coated in this manner would not last through a single casting operation.